I just swapped the tires on my road bike, front to back and back to front, but not for the reasons above. I had a 700x25 in the back that didn't have much clearance between the seat stays and would rub slightly. I swapped it with the 23 in the front. I only bought the 25 because that was all the shop had and I was going to start a 100 mile ride after the old tire was gashed.

For my commuter, I've gotten 3 flats in the last 2 weeks, mostly riding through the same wet roads as Tom does in NYC so I bought a new pair of tires and will replace both.